1. Field of the Invention
This invention relates to a photoresist coating machine, and more particularly to a dispensing system used in a photoresist coating machine.
2. Description of Related Art
Photolithography plays an essential role in semiconductor fabrication. All semiconductor devices need several photolithography process to transfer desired patterns so as to form the devices as designed. A semiconductor device usually includes, for example, a transistor structure with proper doping regions, a capacitor, and an interconnecting structure for connection between each component. All these needs several different photolithography processes. A more complex structure accordingly needs more photolithography processes.
The detailed photolithography technology is usually complicated but its theory is straightforwardly simple. Generally, a photoresist layer is coated on a device substrate surface, on which a device is fabricated. The photoresist layer then is exposed by a light source through a photomask, which carries a pattern to be transferred onto the device substrate surface. The photoresist layer includes photo-sensitive material that can be exposed to light to selectively change its material property. After development, the remaining photoresist layer on the device substrate surface forms the desired pattern. The desired pattern therefore is transferred to the device substrate. Up to this stage, the photolithography process is done. A subsequent process, such as etching or doping, are performed to form one sub-structure of the device, which usually includes several different sub-structures. So, a more complex structure accordingly needs more photolithography processes.
Currently, the photoresist layer can be formed by a spin coating process so as to obtain its thickness uniformity and adhesion without defects. FIG. 1 is a side view of a conventional spinner used for spin coating. A substrate 12 is held by a spinner 10. The spinner 10 fixes the substrate 12 by sucking it with a vacuum force, which is created through a rotating axle of the spinner 10. As the substrate 10 is rotated by the spinner 12, a liquid photoresist 14 sprayed on the substrate 12 is outwardly distributed due to centrifugal force. The liquid photoresist 14 contains volatile organic solution. After volatilization, a uniform photoresist layer 16 with strong adhesion is formed over the substrate 10.
FIG. 2 is a schematic drawing of a conventional photoresist dispensing system included in a photoresist coating machine. In FIG. 2, the photoresist coating machine is, for example, a DNS 636 photoresist coating machine. The conventional dispensing system includes a photoresist cabinet 20 to contain a liquid photoresist 22, which are transported through, for example, a duct 90. A pump 24 is operated to pump out the liquid photoresist 22, which is then transported to a filter 26 and a check valve 28 on the duct 90. The purpose of the check valve 28 is to prevent the liquid photoresist 22 from flowing back. The liquid photoresist 22 passing through a sucking-back valve 30 then is transported to a duct end 46 and is sprayed onto a substrate 34 held by a spinner 32. The sucking-back valve 30 is used to allow the liquid photoresist 22 to be sucked back a little from the duct end 46 when the photoresist coating process completes so that undesired extra liquid photoresist is prevented from dropping on the substrate 34, causing a deterioration of a photoresist layer (not shown) formed on the substrate 34.
The pump 24 and the sucking-back valve 30 are controlled by a solenoid valve 36, which switches operation modes between a dispensing mode and a stopping mode. In the dispensing mode, the liquid photoresist 22 is transported to the duct end 46 to spray the substrate 34. In stopping mode, the liquid photoresist 22 is not dispensed and on the contrary is sucked back a little to prevent undesired photoresist from dropping out onto the substrate 34. The solenoid valve 36 uses an exhaust end EXH and an air-injection end AIR to create an air flow to achieve the purpose of control. The single line route 91 between each components represents an air flow route.
The air-injection end (AIR) is coupled to the pump 24 through an air-flow speed controller SC3. The exhaust end (EXH) is coupled to the sucking-back valve 30 through two air-flow speed controllers SC1, SC2, which are serially coupled. A air-flow speed controller SC4 is coupled between the solenoid valve 36 and the pump 24. The air-flow speed controllers SC1, SC2, SC3, SC4, and SC4 include a regulating valve 38 and an one-way valve 40 shown in speed controller SC2.
When photoresist coating completes, the solenoid valve 36 through a combining actions of the AIR and the EXH on the gas-flow speed controllers SC3, SC4 induces a move of a cylinder 42 inside the pump 24 to stop pumping of the pump 24. Simultaneously, a diaphragm 44 inside the sucking-back valve 30 is pulled back (to the right in FIG. 2). Ideally, the pump 24 stops first so that the liquid photoresist 22 stop supplying. Then, the sucking-back valve 30 produces a sucking-back force to pull the liquid photoresist 22 a little back from the duct end 46 so as to prevent a little undesired liquid photoresist, such as one or two drops, from dropping on the formed photoresist layer (not shown) on the substrate 34.
In practical operation, it is difficult to control the sequential order of actions of stopping the pump 24 and producing sucking-back force from the sucking-back valve 30. If the sucking-back valve 30 is activated before the pump 24 is stopped, extra drops of the liquid photoresist 22 may drop onto the formed photoresist layer on the substrate 34. Moreover, during switching transition period of the solenoid valve 36 from stop to action, if the sucking-back valve 30 releases its sucking-back force before the pump 24 starts operation, extra drops of the liquid photoresist 22 may also drop onto the substrate 34 before photoresist coating operation starts. This also deteriorates the quality of the photoresist layer, which is to be formed.
In summaries, the conventional photoresist dispensing system used to coat a photoresist layer on a substrate has a problem to properly control the action time order of the pump 24 and the sucking-back valve 30. This may cause a few undesired drops of the liquid photoresist 22 to drop onto the substrate 34 and deteriorate the quality of a photoresist layer to be formed.